The photo should show the subject -- and a sign showing the subject's location. On its website, NASA has a sign you can print out and use, but it also suggests you write your location in the sand or with rocks or in some other creative way.

Photo snapped, NASA wants you to then post it to social media. Post to Twitter, Google+ and Instagram with the hashtag #GlobalSelfie. Or post to GlobalSelfie groups on Facebook and Flickr.

The agency will use the pictures, it says, "to create a mosaic image of Earth -- a new "Blue Marble" built bit by bit with photos."

Which the kids likely will find cool -- and can be done without lookinng at the lovely but complicated Earth Day projects posted on Pinterest you know you cannot do before bedtime.